A conventionally proposed driving device for moving a light source unit in, for example, an image reading device or an image forming apparatus includes drive wires connected to the light source unit and drive pulleys around which the drive wires are wound. The driving device operates by driving the drive pulleys so that the drive pulleys rotate and move the light source unit.
FIGS. 11 and 12 are a schematic cross-sectional view and an external perspective view, respectively, of a drive pulley and a drive wire in a conventional driving device similar to the one disclosed in JP 62-154443 U. As illustrated in FIG. 11, a drive wire W, after being fixed in a fixing hole H on a drive pulley P by means of a fixing member B, starts to be wound in a groove G on a surface Ps of the substantially columnar drive pulley P so that the drive wire W is wound on the surface Ps. Because the drive wire W is fixed in the fixing hole H by means of the fixing member B, the winding of the drive wire W starts at a position below the surface Ps of the drive pulley P, somewhat close to the rotation center thereof, and progresses following the groove G so that the drive wire W runs on the surface Ps.
The drive wire W has flexural rigidity in accordance with the material and diameter thereof. In addition, the position of the drive wire W where the drive wire W is fixed using the fixing member B is separated from the surface Ps only by a short distance as illustrated in FIG. 12. For these reasons, the drive wire W fails in some cases to faithfully follow the shape of the surface Ps, forming bulges Wf on the surface Ps near a winding-starting position and at a boundary with the groove G.
If the drive wire W is lifted forming the bulges Wf, when the drive wire W is wound around the drive pulley P by activating the driving device, the lifted portions could disrupt winding of uniform lengths of the drive wire W or the lifted portions could be rewound upon driving and cause the drive wire W to slip. If the drive pulley P winds the drive wire W non-uniformly in this manner, the light source unit, which is a driven body connected to the drive wire W, is likewise moved non-uniformly. This non-uniform motion would undesirably cause positional deviations in image reading and hence blurring in scanned images.
The present invention, conceived in view of these conventional problems, has an object of providing a driving device, an image reading device, and an image forming apparatus each capable of preventing a drive wire from being lifted from a wire-winding surface of a drive pulley in order to reduce irregular winding caused in driving by the drive wire being lifted and to move a driven body at a uniform speed.